1. Field of the Invention
This invention relates to an imaging optical system and an original reading apparatus. More particularly, it relates to an imaging optical system and an original reading apparatus comprising an infrared cut filter arranged in the optical path thereof.
2. Related Background Art
FIG. 1 of the accompanying drawings is a schematic cross sectional view of a known image reading apparatus taken along the main scanning direction thereof. Such an image reading apparatus is typically used for a film scanner.
Referring to FIG. 1, the object to be read (original image) that is a transparency type original (film) 101 is irradiated with a light beam emitted from a fluorescent lamp 102 operating as light source. Then, an image of the illuminated original 101 is formed on a sensor unit 104 by means of an imaging optical system 103 with a predetermined magnification. The sensor unit 104 detects the light beam from the original 101 and outputs an analog signal (analog information) 106. The analog signal is amplified by an operational amplifier (not shown) and input to an A/D converter 105. The A/D converter 105 converts the input analog signal into a digital signal (digital information). The relative position of the original 101 and the sensor unit 104 is shifted by a drive means (not shown). For example, image data may be read from the original 101 by driving the original 101 in the sub-scanning direction that is perpendicular to the main scanning direction of the imaging optical system.
A so-called monolithic 3-line sensor unit comprising three line sensors arranged on a same substrate is typically used for the sensor unit 104. Each of the line sensors is formed by linearly arranging a plurality of photoelectric conversion elements (light receiving pixels) along the main scanning direction. Charge coupled devices (CCDs) are typically used for such line sensors. Filters of three colors of red (R), green (G) and blue (B) are arranged respectively on the three line sensors so that the original 101 may be read in color.
FIG. 2 of the accompanying drawings is a graph showing the spectral sensitivity characteristics of the three line sensors of a monolithic 3-line sensor unit used as sensor unit 104 in the original reading apparatus and the emission spectrum of the fluorescent lamp. Note that the curves are drawn by using a relative scale. In FIG. 2, the horizontal axis shows the wavelength of light while the vertical axis indicates an arbitrarily selected relative scale. The dotted curve 111 indicates the emission spectrum of the fluorescent lamp. The solid curves 112-R, 112-G and 112-B in FIG. 2 indicate spectral sensitivity distribution patterns of the line sensors equipped respectively with color filters of red (R), green (G) and blue (B) filters.
Generally, fluorescent lamps contains mercury and xenon in the inside of the tube because the light emitting mechanism of the tube requires the use of such substances. Then, a fluorescent lamp shows an emission spectrum extending over the wavelength range of not only visible light (between 400 nm and 700 nm) but also infrared rays (700 nm and more). On the other hand, the spectral sensitivity distribution patterns of the line sensors as indicated by the curves 112-R, 112-G and 112-B are determined basically by the spectral sensitivity distribution characteristics of the silicon used in the photoelectric conversion elements and the transmission spectral distribution characteristics of the color filters arranged on the respective line sensors. Thus, the spectral sensitivity distribution characteristics of the three line sensors do not show any significant difference in the wavelength range of infrared rays as seen from the curves 112-R, 112-G and 112-B. This means that a substantially same signal is output from each of the three line sensors for the wavelength range of infrared rays regardless of the information on the color image of the original 101. In other words, signals for the infrared wavelength range is laid on the respective color signals detected for the three color sensors to narrow the dynamic range for reading the color image.
With conventional techniques, this drawback is eliminated and adversely affecting infrared rays are removed by means of an infrared cut filter 107 that is a multilayer film 107 formed on one of the surfaces of a lens of the imaging optical system, for example the light receiving surface 103A of the imaging optical system 103 shown in FIG. 1.
FIG. 3 of the accompanying drawings is a graph showing the spectral characteristics of such a conventional infrared cut filter. In FIG. 3, the horizontal axis indicates the wavelength of light while vertical axis shows the transmittivity of light of the filter. The substrate of the infrared cut filter is made of glass and has a refractive index of 1.52. In FIG. 3, the curve 121 shows the spectral characteristics (equivalent characteristics) of an infrared cut filter prepared by alternately forming a total of twelve layers of high refractive index dielectric layers typically of TiO2 and low refractive index dielectric layers typically of SiO2 or MgF2 on a glass substrate by evaporation. Each of the dielectric layers of this infrared cut filter has a thickness of 225 nm, except the uppermost twelfth layer that is made to have a thickness of 105 nm in order to suppress the ringing effect of the spectral characteristics. Note that the plotted values are those relative to rays of light striking the substrate perpendicularly.
The curve 122 in FIG. 3, on the other hand, shows the spectral characteristics of an infrared cut filter prepared by alternately forming a total of twenty-four layers of high refractive index dielectric layers typically made of TiO2 and low refractive index dielectric layers typically made of SiO2 or MgF2 on a glass substrate by evaporation. Each of the dielectric layers of this infrared cut filter has a layer thickness of 225 nm, except the uppermost twenty-fourth layer that is made to have a thickness of 105 nm in order to suppress the ringing effect of spectral characteristics.
It will be appreciated from FIG. 3 that the infrared cut filter indicated by the curve 121 and comprising twelve layers shows a transmittivity of higher than 1% at and near the wavelength of 900 nm where the filter is least transmissive to prove a relative poor performance of the filter. For the convenience of explanation, the wavelength zone where the transmittivity of a filter is less than 10% is referred to as effective infrared cut zone of the filter. Then, the effective infrared cut zone of the infrared cut filter having twelve film layers is found between 810 nm and 1,030 nm, which is narrow and not satisfactory for practical applications. Particularly, the infrared cut filter of the curve 121 is ineffective for cutting near infrared rays that are close to rays of visible light.
On the other hand, the infrared cut filter indicated by the curve 122 in FIG. 3 and comprising twenty-four layers shows a transmittivity of about 0.1% at and near the wavelength of 900 nm where the filter is least transmissive to prove a satisfactory performance of the filter. However, the effective infrared cut zone of the filter is found between 790 nm and 1,040 nm, which is also narrow and not satisfactory for practical applications. Additionally, the large number of dielectric layers of this filter may constitute a manufacturing problem. Still additionally, an infrared cut filter having such a large number of layers may be liable to show peeled layers with time particularly under adverse environmental conditions in terms of temperature and humidity.
In view of the above identified problems of the prior art, it is therefore the first object of the present invention to provide an imaging optical system having a simple configuration that can effectively and satisfactorily remove infrared rays over a wide wavelength zone.
The second object of the present invention is to provide an image reading apparatus that is free from the above identified problems of the prior art and can read an image with a wide dynamic range by effectively and satisfactorily removing infrared rays.
According to the invention, the above first object is achieved by providing an imaging optical system comprising:
a plurality of lenses; and
infrared cut filters arranged on at least two of the respective surfaces of the plurality of lenses, said infrared cut filters showing respective spectral characteristics that are different from each other.
According to the invention, the above second object is achieved by providing an image reading apparatus comprising:
a light source for illuminating an original image;
an imaging optical system for forming an image of the illuminated original image on the focal plane of the system;
a sensor unit arranged in front of said focal plane; and
at least two infrared cut filters arranged on the optical path between the light source and the sensor unit, said infrared cut filters showing respective spectral characteristics that are different from each other.
According to the invention, there is also provided an image reading apparatus comprising:
a light source for illuminating an original image;
an imaging optical system for forming an image of the illuminated original image on the focal plane of the system;
a sensor unit arranged in front of said focal plane;
an A/D converter for converting the output signal of said sensor unit into an n-bit digital signal; and
an infrared cut filter unit arranged on the optical path between the light source and the sensor unit, said infrared cut filter having a wavelength zone satisfying the relationship as expressed by the formula below:
Txe2x89xa6100/{2**(nxe2x88x921)},
where T is the transmittivity of said infrared cut filter and ** is the power exponent.